1. Field of the invention
The present invention is generally concerned with brakes, usually for industrial use, of the kind comprising two pivoting arms adapted to apply brake pads to a member to be braked, a disk, for example, and a piston-and-cylinder actuator with one member, either the cylinder or the piston, coupled to one of said arms and the other member coupled to the other arm, together with brake application means connected to one chamber of said actuator to urge its piston in one direction and brake release means connected to the other of said chambers to urge said piston in the opposite direction.
2. Description of the prior art
A brake of this kind is described in published French patent application No 2 598 763 filed 16 May 1986.
In this brake the release means constitute a pressure generator comprising a pump to feed pressurized fluid to the corresponding chamber of the actuator commanded as required by a motor. The pump flow is controlled by a pressure-operated switch.
When the brake is released, the corresponding chamber of the actuator, previously vented, is supplied with fluid under pressure by this pump and the pressure in it rises gradually until it balances the oppositely directed force exerted by the associated application means.
The piston of the actuator then moves in a direction corresponding to releasing the brake, until it comes into contact with an abutment.
From this moment the pressure in the chamber of the actuator concerned begins to rise again to a maximal value at which the pressure switch stops the pump.
As leaks are inevitable in the corresponding circuit, the pressure in this chamber then begins to fall gradually.
If it reaches a minimal threshold value the pressure switch restarts the pump.
Thus this brake is held open (released) by a pressure switch operating in a cyclic manner associated with a pressure difference between a maximal pressure and a restarting threshold.
This arrangement has given satisfaction and may continue to do so.
It has the following disadvantages, however.
First of all, when it is required to be reliable, as is obviously the case in this application, a pressure-operated switch is relatively costly, which is prejudicial to the overall unit cost.
This disadvantage is aggravated when the pressure switch must be explosion-proof so that it can be used in a potentially explosive atmosphere.
Also, the necessity for the piston of the actuator to come into contact with an abutment means that its travel may be greater than that strictly necessary to release the brake and this superfluous movement inevitably results in some wear of the actuator.
What is more, because of its relatively low sensitivity a pressure switch requires a relatively high threshold for restarting the pump, this threshold being related to the pressure difference for which the pressure switch must operate.
The result is that the motor is started relatively frequently, imposing relatively high fatigue stresses on the pressure generator as a whole and causing significant overheating, prejudicial to durability.
Another result is the risk of excessive pressure at the actuator piston seal, which is prejudicial to safety.
Finally, it is necessary to provide means for systematically compensating wear of the brake pads, failing which the closing travel of the brake pads and therefore the response time of the brake would increase to an unacceptable degree as such wear proceeded.
At present these wear compensating means are usually mechanical means using a chock and a screw to form an abutment. Complicated, and therefore relatively costly, such systems are ill adapted to accommodating possible axial displacement and/or warping of the disk, with the attendant risk of continuous contact between the brake pads and the disk leading to the risk of unacceptable overheating of the disk or even unacceptable deterioration of the brake as a whole.
A general object of the present invention is an arrangement for circumventing these disadvantages which confers additional advantages.